1. Field of the Invention
The present invention relates to an image sensing system and a control method therefor, and more particularly, to an image sensing system for driving a solid-state image sensor composed of a CMOS image sensor including a column circuit, and a control method therefor.
2. Description of the Related Art
Conventionally, CMOS image sensors have been known as XY addressing type image sensors which are capable of reading out electrical charges for each pixel. It is to be noted that the CMOS is an abbreviation of a Complementary Metal-Oxide Semiconductor. CMOS image sensors have advantages such as their smaller power consumption, lower driving voltage, and the ability to speed up charge readout, as compared with CCDs of conventionally mainstream image sensors.
Furthermore, CMOS image sensors allow on-chip peripheral circuits, and CMOS image sensors have been thus developed which include on-chip circuits for each column of pixels arranged in a matrix. For example, various techniques have been proposed, including a technique for achieving high S/N with on-chip amplification circuits for each column, and a technique for achieving speeding up by A/D conversion using on-chip A/D converters for each column.
On the other hand, differences in characteristics among the on-chip circuits provided for each column are caused due to variations in manufacturing process, causing vertical-stripe fixed pattern noise in shot images. It is to be noted that such noise caused by differences in circuit characteristics for each column is referred to as column noise.
Japanese Patent Laid-Open No. 2000-261730 discloses a technique for reducing such column noise. More specifically, in Japanese Patent Laid-Open No. 2000-261730, a vertical black reference region shielded from light is provided separately from an effective pixel region, and output signals from the vertical black reference region are used to acquire column noise. Then, the acquired column noise is stored as digital values, and the stored digital values are used to correct shot image signals in the effective pixel region, thereby allowing favorable images with reduced vertical-stripe column noise to be obtained.
However, according to the method described in Japanese Patent Laid-Open No. 2000-261730, high-accuracy correction data is required in order to correct column noise correctly, and the method thus has a problem in that a region comprised of a number of pixels for acquiring the correction data is required separately from an effective pixel region, because much more pixel data needs to be averaged in the vertical direction in order to sufficiently reduce random noise during correction data acquisition.
Furthermore, in a case in which it is not possible to sufficiently reduce random noise during correction data acquisition, or in a case in which the pixel region for acquiring correction data has defects and the like, the accuracy of acquired correction data will be lowered. In a case in which such low-accuracy correction data is used to carry out correction, there is a problem in that vertical-stripe noise is caused rather than reduced.
Moreover, in a case in which output signals from pixels shielded from light are used to acquire correction data, there is a problem in that the correction data accuracy is lowered, for example, in a case in which there are abnormal pixels such as pixel defects, or due to the effect of unevenness and the like resulting from dark current.